Peer-to-peer car sharing network RelayRides went live this morning with the ability for all 6 million subscribers to GM’s OnStar connected car service to rent out their cars. This is a big win for RelayRides, as putting supply into the market can be a real challenge, particularly when you’re convincing car owners to rent their vehicles to strangers. If only 1 percent of OnStar subscribers offer up their cars for renting witin the RelayRides network, that would be 60,000 cars, more than six times as many cars as Zipcar offers. If 0.1 percent offer up their cars, that’ll put RelayRides close to even with Zipcar in terms of available cars. GM’s venture arm invested in RelayRides and GM says it plans to up the API for OnStar to other third party developers.